Tenth-value-layers (TVLs) for a 6 MV stereotactic radiosurgery (SRS) x-ray beam have been computed using Monte Carlo methods for radiation transport simulation. The first and equilibrium TVLs were determined in the three most common building materials used in radiation therapy vault construction: ordinary concrete, lead, and steel (iron). In contrast to broad-beam 6 MV TVL data found in the literature, the SRS TVLs can change rapidly with the size of the radiation field incident on the barrier. This research has investigated characteristics of TVLs as a function of field size (diameter) at the barrier for all materials, with special attention given to the TVL properties in iron. The x-ray spectrum used to perform these simulations was generated for the CyberKnife accelerator with the BEAMnrc Monte Carlo code. Using this spectrum as input to the MCNP5 Monte Carlo code, predicted tissue-maximum-ratio (TMR) values for a 6-cm-diameter field (at 80 cm from the target) were benchmarked against measured TMR data. The MCNP5 code was used to simulate all barrier transmissions, keeping the standard error of each data point below 1% of the mean. Results compare very well with previous measured concrete TVLs and also with published broad-beam 6 MV TVL data for all three barrier materials.